The invention relates to the field of optical reflectors, and in particular to fabricating broadband light reflectors and waveguides based on high index contrast materials such as Si/SiO2.
Light reflectors have a broad application spectrum in laser optics and integrated optics. They are part of laser mirrors as well as more elaborate devices such as output couplers, optical filters and dispersion compensators. In integrated optics they can be used for guiding waves in waveguides. The state of the art in producing light reflectors employs either metal reflectors, dielectric Bragg reflectors or semiconductor Bragg reflectors.
Metal reflectors consist of a thin metal layer such as gold, aluminum or silver that is deposited on a glass, semiconductor or similar substrate. They reflect light over a very broad range of wavelengths, but their reflectivity is not very high (e.g., <98%) due to absorption in the metal. In addition, light of different wavelengths undergoes different phase shifts when reflected from the metal, which results in a dispersive reflection. In addition, metal reflectors are not transparent, which prohibits their use in output couplers and beam splitters. When a metal reflector is to be deposited on a semiconductor optical element, for example a saturable absorber, an additional process step beyond the growth of the semiconductor element is required for its deposition. An additional dielectric coating can be employed to enhance the reflectivity of the metal reflector or to protect its surface. This coating can be deposited on top of the metal film. However, for a large number of applications, loss and dispersion restrict the use of metal reflectors.
Dielectric reflectors can be used in many places where reflectivity and dispersion characteristics prohibit the use of metal reflectors. They consist of a coating of dielectric layers of alternating refractive indices, e.g. SiO2 and TiO2, which is deposited on a glass or semiconductor substrate. At each boundary between two adjacent layers a small fraction of the light is partially reflected, such that the coherent superposition of the many partial reflections results in a reflecting element. The partial reflection of light at the layer boundaries is determined by the refractive indices of the two adjacent layers. It is thus a property of the material system selected and largely determines some of the most important properties of the reflector: reflectivity, bandwidth and angle of acceptance. The larger the contrast of refractive indices at each layer boundary, the larger the partial reflection in this place becomes, and as a result, the smaller the number of total layers becomes, that is needed to achieve a certain overall reflectivity.
The commonly used dielectric material system SiO2/TiO2 (indices n=1.45 and 2.3) has a partial reflectivity of 5% at each index discontinuity, and 11 layer pairs are needed for a 99.8% high reflector. In contrast, a mirror fabricated with Si/SiO2 (indices n=3.5 and 1.45) has a three times larger partial reflection and achieves the same overall reflectivity with only six layer pairs. Furthermore, the index contrast of the layer materials determines the bandwidth of the reflector and the acceptance angle for light to be reflected. Here it is again desirable to have an index contrast as large as possible to achieve a large bandwidth and acceptance angle.
For a given index contrast, the reflectivity can be increased by adding more layers and to a certain extent the bandwidth of the reflector can be extended by a technique called chirping that is well known in the art. The same holds for the design of reflective optical filters such as narrowband reflectors or dispersion compensators, where the properties of the device to be designed can be improved by an increasing number of layers. This problem is comparable to digital filter design, where an increasing number of filter coefficients allows to get closer to a design target. However, limits to this approach of an increasing number of layers are given by fabrication cost, by finite fabrication tolerances and the fact that the layer thicknesses are the more difficult to control during fabrication the thicker the coating becomes. In addition, stress, strain and adhesion of the reflective coating to its substrate limit the maximum coating thickness.
Thus, a larger index contrast of the layer materials allows one to achieve the same performance characteristics with a smaller number of layers or it allows for reflector characteristics that can not be possible with small-index materials. While dielectric reflectors are easily deposited on glass substrates, their deposition on semiconductor materials can lead to problems resulting from stress, low adhesion and mismatch of thermal expansion. In addition, the deposition of dielectric reflectors on semiconductors and semiconductor elements always involves additional process steps in a different deposition machine. This added complexity is very undesirable.
For that reason, in some semiconductor elements such as semiconductor saturable absorbers, reflectors are fabricated out of semiconductor materials. In the class of frequently employed III–V materials, this approach suffers from the problem, that the index contrast of the materials is very small, leading to a narrow bandwidth and narrow acceptance angle and requires a large number of layers to achieve a certain bandwidth.
Furthermore, a large angle of acceptance for the incident light to be reflected is desirable. This property is especially important when the reflector is integrated with other semiconductor devices for example as a substrate in integrated optics or as a backmirror for light emitting devices or solar cells.
Omnidirectional reflectors, i.e., reflectors that reflect light incident from all possible directions and at all polarizations, have been demonstrated recently. If the index contrast of the reflector is high enough, and if the low-index layer has an index that is higher by about a factor of 1.5 from that of the incident medium, a reflector becomes omnidirectional for a certain wavelength range. These conditions are satisfied for a Si/SiO2 mirror with air as the incident medium over a bandwidth of approximately 25% of the center wavelength. For SiO2 as incident medium (n=1.45) and SiN, Ta5O4, Ti (n=2.0 . . . 2.2) as low-index layer and Si (n=3.5) as high index layer there is a very small wavelength region around the center wavelength of the reflector, where it is omnidirectional. Such a reflector based on the Si/SiO2 material system has been used in the prior art. Alternating layers of high refractive index (Si) and low refractive index (SiO2) are deposited on top of a silicon substrate.
If the Si and SiO2 layers are deposited on top of the wafer layer by layer either by CVD processes or sputtering or a similar deposition process, imperfections in the surface quality accumulate such that the topmost layer has a significantly roughened surface. The surface roughness increases, the more layers are deposited. Thus, it is highest at the top of the deposited multilayer stack. Here, close to the surface of the mirror. Since it is at the position of this layer that the strength of the electric field is highest, and thus, the accumulated surface roughness has the largest impact. It leads to significant scattering loss, especially in the short-wavelength range of the stop band of the mirror. This problem is especially pronounced in Si/SiO2 and similar high index contrast material systems. Due to the scattering loss the application of mirrors produced according to this process is limited to applications where higher loss can be tolerated. In contrast, in more narrowband dielectric multilayer stacks this problem does usually not arise.
If, in contrast, the layers are not deposited by CVD processes but added on top of each other by wafer bonding, high quality surfaces are present throughout the device. Thus, scattering losses will be minimal. However, due to the large number bonding steps or repeated application of smart-cut processes the fabrication process has a significantly reduced yield. The wafers have to be processed one by one, and in each bonding step a certain number of wafers break or turn out to be useless. This process has been demonstrated in prior art.